In generally producing polished glass products which are required to have a high degree of surface flatness and in-plane uniformity, such as a liquid crystal display (LCD) glass substrate, a hard, disk (HDD) glass substrate, a recording device glass disk, an optical lens, a silicon wafer, a semiconductor device, and the like, first a “rough cutting step” using a polishing pad made of an urethane resin is performed as a first step, and then a “finishing step” using an abrasive cloth for fine polishing is performed as a second step.
As a general method for producing the polishing pad made of a urethane resin, so far, there have been proposed various methods, for example, using a so-called two-component urethane resin composition including a combination of a curing agent and a base resin which contains an isocyanate group-terminated urethane prepolymer.
As a polishing pad using such a two-component polyurethane resin composition, for example, a polishing pad having a polishing layer composed of a polyurethane resin foam having fine foam cells is known, in which the average cell diameter of the fine foam cells is 20 to 70 μm, a rate of thermal dimensional change of the polishing layer is 3% or less, the polyurethane resin foam contains an isocyanate component and a polyol component as raw material components, the isocyanate component contains an aromatic diisocyanate and an alicyclic diisocyanate, the content of the polyol component in the polyurethane resin foam is 15 to 55% by weight, the aromatic diisocyanate is toluene diisocyanate, the alicyclic diisocyanate is dicyclohexylmethane diisocyanate, the polyol component is a polyether polyol having a number-average molecular weight of 500 to 5000, the polyurethane resin foam contains an isocyanate-terminated prepolymer containing an isocyanate component, a polyol component, and a low-molecular-weight polyhydric alcohol, and a chain extender, and the average number of functional groups of the polyol component and the low-molecular-weight polyhydric alcohol is 2.0 to 2.8. In this polishing pad, the rate of thermal dimensional change of the polishing layer during use can be relatively easily controlled to 3% or less by using the foam, and even when a temperature rise occurs due to frictional heat generated in a polishing process, deterioration in flattening characteristics can be decreased as compared with conventional polishing pads (refer to, for example, Patent Literature 1).
However, when a work piece is polished using the polishing paid composed of the two-component polyurethane composition described in Patent Literature 1, the polishing pad having low hardness has low elasticity, and thus the polishing pad composed of the two-component polyurethane resin composition has the problem that settling occurs due to the pressure generated between the polishing pad and the work piece, and a change in hardness (elastic modulus) occurs during a polishing work, thereby causing difficulty in use for uniform polishing of glass products required to have a high degree of surface flatness and in-plane uniformity.
In addition, a polyol composition for a two-component curable foam grindstone is known, in which (A) a polyaminochlorophenylmethane mixture containing 50 to 70% by weight of a binuclear polyaminochlorophenylmethane compound, 20 to 40% by weight of a trinuclear polyaminochlorophenylmethane compound, and 5 to 10% by weight of a tetranuclear or higher-nuclear polyaminochlorophenylmethane compound is uniformly dissolved in (B) a polyol so that an (A)/(B) ratio by weight is 30/70 to 60/40, the polyol (B) being a polyol having a molecular weight of 100 to 1500 and an ether bond in a main chain and/or a polyol having a molecular weight of 50 to 500 and a methyl group in a side chain. Such a polyol composition for a two-component curable foam grindstone includes specified amounts of the polyaminochlorophenylmethane compound (A) and the polyol (B) and thus shows good compatibility, good dissolution stability, and liquid properties, whereby a foam for a grindstone can be molded with a simple two-component mixing-casting machine, water as a foaming agent can be added to a polyol composition containing 3,3′-dichloro-4,4′-diaminodiphenylmethane (MBOCA) as an amine curing agent, and the temperature of the composition for a two-component curable foam grindstone can be controlled to the boiling point of water or less, thereby producing a foam grindstone having a uniform density distribution and good mechanical physical properties because water is not scattered during molding and causing usefulness for polishing fine glass products of glass, semiconductor silicon, and the like (refer to, for example, Patent Literature 2).
However, the polyol composition for a two-component curable foam grindstone described in Patent Literature 2 has the problem that it is embrittled and made easy to wear by increasing the MBOCA amount in the curing agent, leading to a decrease in life of a polishing pad, moldability deteriorates clue to an increase in reactivity, and physical properties such as compression set and the like deteriorate, thereby causing difficulty in use for uniform polishing of glass products and the like which are required to have a high degree of surface flatness and in-plane uniformity.
In addition, a two-component composition for a glass polishing polyurethane pad composed of a base resin (a) containing an isocyanate group-terminated urethane prepolymer (A), and a curing agent (b) containing an isocyanate group reactive compound (B), water (C) as a foaming agent, an inorganic polishing agent (D), and a catalyst (E) is known, in which the isocyanate group-terminated urethane prepolymer (A) is an isocyanate group-terminated urethane prepolymer produced by reacting at least a tolylenediisocyanate (A1), poly(tetramethylene ether) glycol (A2), and polycaprolactonetriol (A3) so that when (A2) and (A3) are mixed, an average number of functional groups falls in a range of 2.1 to 2.7, and the isocyanate group reactive compound (B) is composed of at least 4,4′-diamino-3,3′-dichlorodiphenylmethane (B1) and poly(tetramethylene ether) glycol (A2). In this two-component composition for a glass polishing polyurethane pad, the prepolymer has low viscosity, bubble biting and air voids during mixing and molding can be decreased to form uniform foam cells, and the average number of functional groups of the polyol mixture can be easily optimized to suppress decrease in hardness of the polishing pad due to the heat generated during polishing (refer to, for example, Patent Literature 3).
However, when a work piece is polished using a polishing pad composed of the two-component composition for a glass polishing polyurethane pad described in Patent Literature 3, the polishing pad composed of the two-component composition with low hardness has low elasticity and thus has the problem that settling occurs due to large frictional heat generated between the polishing pad and the work piece and pressure generated between the polishing pad and the work piece, thereby causing a change in hardness (elastic modulus) during a polishing work and causing difficulty in use for uniform polishing of glass products and the like which are required to have a high degree of surface flatness and in-plane uniformity.
Further, a polishing pad for Cu-film polishing which has a polishing layer composed of a polyurethane resin foam is known, in which the polyurethane resin foam is a reaction cured product of an isocyanate-terminated prepolymer containing an isocyanate component and a high-molecular-weight polyol component as raw material components and a chain extender, and the high-molecular-weight polyol component contains 30% by weight or more of polyester polyol. The polishing pad for Cu film polishing uses polyester polyol which is a polymer of an oxyethylene derivative and adipic acid and thus has good balance between higher elasticity and hydrolyzability of the polyurethane resin foam and the excellent effect of preventing thinning, dishing, and scratch (refer to, for example, Patent Literature 4).
However, the polishing paid for Cu film polishing described in Patent Literature 4 has the problem remaining unresolved that settling occurs due to pressure generated between the polishing pad and the work piece, thereby causing difficulty in use for uniform polishing in Cu film polishing which is required to have a high degree of surface flatness and in-plane uniformity.
As described above, for the purpose of improving surface flatness and in-plane uniformity, it has been desired to develop a two-component urethane resin composition for a polishing paid which has particularly high hardness (high elasticity), low permanent set, and excellent control of brittleness, fine cell control, and moldability (yield, molding irregularity), and to develop a polyurethane polishing pad and a method for producing a polyurethane polishing pad.